1. Field of the Invention
The present invention relates to structuring of system resources for allocation to a user of a multi-access telecommunication system. In particular, the present invention relates to structuring of frequency resources for allocation to a user in a multi-access telecommunication system, in response to changing channel conditions (i.e., a “time-varying channel”).
2. Discussion of the Related Art
A strong signal path is essential for reliable communication. In a channel having rapidly changing conditions, a signal path may experience a deep fade. When a signal path experiences a deep fade, the bit-error rate is high. Diversity techniques, such as transmitting the symbols over multiple signal paths, improve performance in fading channels. When the signal paths fade independently, reliable communication is maintained when at least one of the signal paths remains strong.
There are many ways to achieve diversity. For example, U.S. Pat. No. 5,457,712, entitled “Method for providing time diversity” (“Weerackody”) to V. Weerackody, issued on Oct. 10, 1995, discloses that temporal diversity may be achieved using interleaving and error-correcting codes. Weerackody proposes dispersing the coded symbols over time in different coherence periods, so that different parts of the symbols experience independent fades.
Similarly, U.S. Pat. No. 6,788,751, entitled “Frequency Diversity Digital Wireless System” (“Hustig”) to C.H. Hustig et al., issued on Sep. 7, 2004, discloses frequency diversity techniques in frequency-selective channels, such as coded-orthogonal frequency division multiplexing (C-OFDM), multiband-OFDM (MB-OFDM), and using a rake receiver.
In addition, U.S. Pat. No. 6,853,694, entitled “Spatial diversity wireless communications (radio) receiver” (“Beandin”) to A. Beaudin et al., issued on Feb. 8, 2005, discloses a spatial diversity technique which uses multiple transmit or receive antennas that are spaced sufficiently far apart. U.S. Pat. No. 6,104,933, entitled “Method and apparatus for control of base stations in macro diversity radio systems” (“Frodigh”) to M. Frodigh et al., issued on Aug. 15, 2000, discloses macro diversity in a cellular network. In Frodigh, macro diversity is achieved by receiving the signal from a mobile station (MS) at two base stations (BSs). Frodigh's technique may be implemented, for example, in multiuser multiple-input-multiple-output (MU-MIMO), network-MIMO (NW-MIMO) systems.
More recently, multiuser diversity techniques are disclosed, for example, in the articles (a) “Information capacity and power control in single-cell multiuser communications,” by R. Knopp and P.A. Humblet, Proc. IEEE Intl. Conf. Commun., Seattle, Wash., June 1995, pp. 331-335; (b) “Optimal power allocation over parallel Gaussian broadcast channels,” by D.N.C. Tse, in Proc. Intl. Symp. Inf. Theory, Ulm, Germany, June 1997, pp. 27-27; and (c) “Opportunistic beam-forming using dumb antennas,” by P. Viswanath, D.N.C. Tse, and R. Laroia, IEEE Trans. Inform. Theory, vol. 48, no. 6, June 2002, pp. 1277-1294. Another example of multiuser diversity technique is disclosed in U.S. Patent Application Publication 2007/0064780, entitled “Channel quantization for multiuser diversity” (“Zheng”) by J. Zheng et al. published on Mar. 22, 2007. Zheng discloses quantizing channel state information (CSI) into quantization levels in a finite-rate feedback multiuser system, using performance metrics, such as signal-to-noise ratio (SNR), bit error rate (BER), or system capacity. Other examples of multiuser diversity techniques include (a) U.S. Patent Application Publication 2006/0116077, entitled “Exploiting multiuser diversity through phase modulation multiplexing,” by H. Liu, M. Shen, G. Xing, published on Jun. 1, 2006, which discloses a method for combining signals of multiple users onto a common channel using phase modulation multiplexing, and (b) U.S. Patent Application Publication 2006/0120395, entitled “Method and system for switching antenna and channel assignments in broadband wireless networks” (“Xing”), by G. Xing, M. Shen, H. Liu, published on Jun. 8, 2006, which discloses a method and apparatus for antenna switching, grouping, and channel assignments. Xing proposes using simple antenna operations to increase the capacity and performance of wireless communications systems.
Diversity techniques are such powerful tools that a typical wireless system may use multiple forms of diversity to achieve the intended performance.
Wireless systems that operate under the orthogonal frequency division multiple access (OFDMA) scheme enjoy good resource resolution in the time-frequency grid. Better diversity is expected to be achieved under an OFDMA system, given proper resource allocation and resource structuring. Resource allocation or sub-carrier and power allocation for OFDMA systems are disclosed, for example, in the following U.S. patents or U.S. patent application publications: (a) U.S. 2005/0265223, entitled “Method and apparatus for scheduling downlink channels in an orthogonal frequency division multiple access system and a system using the same,” by J.-H. Song, A1, published on Dec. 1, 2005; (b) U.S. 2006/0109865, entitled “Method for allocating resources in a multicarrier system and transmission apparatus using the same,” by S.-Y. Park, Y.-W. Lee, S.-B. Yun, Y.-S. Kim, published on May 25, 2006; (c) U.S. Pat. No. 6,917,812, entitled “Air interface scheduler for wireless communication networks,” to A. Damnjanovic, issued on Jul. 12, 2005; (d) U.S. Pat. No. 6,987,738, entitled “Method for packet scheduling and radio resource allocation in a wireless communication system,” to V.G. Subramanian, R. Agarwal, R.J. La, issued on Jane 17, 2006; (e) U.S. 2006/0149620, entitled “Scheduling apparatus and method in a multicarrier communication system,” by C.-H. Suh, S.-H. Park, S.-H. Yoon, S.-K. Hong, Y.-K. Cho, published on Jul. 6, 2006; (f) U.S. 2008/0076438, entitled “Method for dynamic resource allocation of uplink and downlink in OFDMA/TDD cellular system,” K.H. Chang, S.J. Ko, T.H. Sun, J.H. Kim, published on Mar. 27, 2008; and (g) U.S. 2008/0043610, entitled “Multi-carrier communications with group-based subcarrier allocation” (“Li”), by X. Li, H. Liu, H. Yin, G. Xing, F. Mu, published Feb. 21, 2008. Li discloses a method for subcarrier selection in an OFDMA system which partitions subcarriers into groups of at least one cluster of subcarriers for use in communication with the subscriber1. 1 Subscriber is also known as MS or user equipment (UE).
None of the OFDMA systems discussed above takes advantage of multiuser diversity, nor do they address time-varying channel conditions. The following publications disclose using multiuser diversity to develop resource allocation or scheduling in schemes that assume perfect or partial knowledge of channel conditions (e.g., quasi-static or very slowly varying channel conditions). (a) the article “Opportunistic beamforming and scheduling for OFDMA systems” (“Svedman”) by P. Svedman, S.K. Wilson, L.J. Cimini, and B. Ottersten, IEEE Trans. Commun., vol. 55, no. 5, May 2007, pp. 941-952; (b) the article “Adaptive resource allocation for multiuser MIMO/OFDM networks based on partial channel state information” (“Chemaly”), by R. Chemaly, K. Letaief, and D. Zeghlache, Proc. IEEE GLOBECOM 2005, St. Louis, Mo., November 2005, pp. 3922-3926; (c) U.S. Patent Application Publication 2007/0110003, entitled “Subcarrier allocation in OFDMA with imperfect channel state information at the transmitter” (“Tujkovic I”), by D. Tujkovic, A. Paulraj, published May 17, 2007; and (d) U.S. Patent Application Publication 2008/0002619, entitled “Time domain interference averaging with multiuser diversity in OFDMA systems” (“Tujkovic II”), by D. Tujkovic and A. Paulraj, published on Jan. 3, 2008. Tujkovic I discloses, for example, a method that combines multiuser diversity and frequency diversity to a resource allocation scheme. The method benefits from multiuser allocation by assigning a fraction of the available bandwidth to users in high SNR channels. Recognizing that CSI at the transmitter is not perfect, the system and method in Tujkovic I allocate the remaining bandwidth pseudo-randomly according to a frequency diversity scheme.
In Tujkovic II, interference among users of an OFDMA system operating under multiuser diversity within a coherence bandwidth is reduced by spreading out the users' transmission symbols randomly in time within the coherence bandwidth. When transmission symbols are randomly dispersed, the average variance of interference among users in the same sub-band is reduced.
In addition to Chemaly (discussed above), the following articles incorporate time-variation or Doppler spread in their resource allocation: (a) “Sub-band rate and power control for wireless OFDM systems” (“Oh”), by J. Oh and J.M. Cioffi, Proc. IEEE Vehicular Technology Conference (VTC 2004-Fall), Los Angeles, Calif., Vol. 3, 26-29 Sep. 2004, pp. 2011-2014; and (b) “Fair adaptive radio resource allocation of mobile OFDMA” (“Chu”), by F.-S. Chu and K.-C. Chen, Proc. IEEE Personal, Indoor and Mobile Radio Communications (PIMRC 2006), Helsinki, Finland, September 2006, pp. 1-5. Chemaly, Oh and Chu each require knowledge of the Doppler spread. Even with knowledge of Doppler spread, these methods still suffer from performance degradation at high mobile speeds. The mismatches between the models used in these methods and the actual ones for the channel time-correlation model and the exact maximum Doppler shift may introduce additional performance degradation. Therefore, while Chemaly, Oh and Chu incorporate time-variation, these methods require the knowledge of the Doppler spread and suffer from performance degradation at high mobile speeds.
Several user resource structures or channelization have been disclosed that take advantage of diversity. However, the disclosures differ in the terminology used. For example, band division multiple access (BDMA) is disclosed in the article “BDMA testbed-configuration and performance results,” by T. Kunihiro, T. Yamaura, M. Suzuki, E. Fujita, Proc. IEEE Vehicular Technology Conference, VTC Spring, Vol. 3, 16-20 May 1999, pp. 1836-1840. Other disclosures describe interleaved frequency division multiple access (IFDMA), adaptive FDMA (AFDMA), and interleaved-type, band type, or distributed structure and localized structure. In addition, the article “Resource allocation and power control in a TDD OFDM-based system for 4G cellular networks,” by P. Bisaglia, S. Pupolin, D. Veronesi, M. Gobbi, Proc. IEEE Vehicular Technology Conference, VTC Spring, Vol. 4, 7-10 May 2006, pp. 1595-1599, discloses adaptive block division multiple access (ABDMA). The article “Performances of multicarrier system with time and frequency domain spreading for wireless communications,” by Y. Teng, K. Naito, K. Mori, H. Kobayashi, Proc. International Conference on Wireless Networks, Communications and Mobile Computing, Vol. 1, 13-16 Jun. 2005, pp. 558-563, discloses non-frequency scattering and hopping (NFSH), frequency scattering (FS), frequency scattering and hopping (FSH), WiMAX and IEEE 802.16e standards introduce concepts such as distributed permutation and contiguous/adjacent permutation.
Despite the large number of terms used, user resource structures may be classified into two main categories: band-type (a.k.a. localized-structure) and interleaved-type (a.k.a. distributed-structure). In a band-type resource structure, a band of subcarriers are allocated to a user. In an interleaved-type resource structure, however, the subcarriers allocated to a user may spread out across a band and interleaved with subcarriers allocated to other users. In an interleaved type, a user may be allocated non-contiguous subcarriers, or several non-adjacent groups of a few contiguous subcarriers, which are spread out across an entire band. Under a more general scheme, the assigned subcarriers may shift from symbol-to-symbol in a systematic way across all users. In general, different user resource structures have different advantages and disadvantages, based on such factors as application usage, channel condition, and mobility scenario.
To provide multiuser diversity in an OFDMA systems, subcarriers assigned to a user should preferably be contiguous (i.e., band-type). When a channel varies substantially over the duration of a transmission frame, the band-type resource structure loses most multiuser diversity gains because of the outdated/mismatched channel information assumptions made for the resource allocation. Under such conditions, a channel allocated to a user may be in deep fade at a later part of the frame. To alleviate the effects of a deep fade, a shorter frame length and a more frequent channel information update (feedback) may be used. However, a substantial throughput loss resulting from a large overhead may become unacceptable. Typically, each frame contains overhead information such as a preamble and control information (e.g., DL-MAP, UL-MAP). Thus, a short frame length is inefficient from an overhead viewpoint.
To operate in a rapidly changing channel, another approach keeps a user's assigned subcarriers spread out over an entire band (i.e., interleaved-type). Such an approach is disclosed, for example, in the article “A new ranging method for OFDMA systems” (“Fu”) X. Fu, Y. Li, and H. Minn, IEEE Trans. on Wireless Commun., vol. 6, no. 2, pp. 659-669, February 2007. Fu's approach addresses the deep fade problem using frequency diversity. Multiuser diversity gain is lost, however. Thus, the existing user resource structures (both band-type and interleaved-type) have limited diversity exploitation capability in a changing channel.
In summary, band-type resource structure requires frequent feedback of channel information to maintain the multiuser diversity gains; such frequent feedback is not practical in a rapidly changing channel. Interleaved-type resource structure is more robust in rapidly changing channels because of frequency diversity gain; such resource structure provides little multiuser diversity gain. Both interleaved-type and band-type user resource structures have limited diversity exploitation capability, especially in rapidly changing channels.